In measuring basis weight or moisture of moving sheet material a radiation source of either, for example, ionizing radiation or infrared is mounted on one side of the sheet material and opposite on the other side is a receiving transducer. The pairs of transducers are then scanned in a cross-direction (perpendicular to the moving or machine direction of the sheet material) to provide a cross-direction profile of that particular characteristic. Due to mechanical imperfections of the transverse mechanisms and the sensitivity of the measuring transducers themselves, a constant positional relationship both in a Z direction, which is perpendicular to the moving sheet, and X, Y directions which are in the plane of the moving sheet are not always maintained. Thus, the characteristic measurements are affected by these relative movements of the transverse mechanisms.
One form of correction is illustrated in Davis U.S. Pat. No. 3,306,103 where screws are preset for each zone or slice of the sheet material to compensate for expected errors. FIG. 2 of Davis illustrates a pair of scanning heads 13 and 14 having a pair of transducers mounted in opposed heads; namely, a radiation source and ionization chamber for determining basis weight. FIG. 5 of the present invention shows similar units 31 and 32. As Davis points out, a change in the positional relationship of the source and the chamber will cause measurement error (Column 1, lines 23-27).
In Watson U.S. Pat. No. 4,276,480 changes in relative relationship are sensed by measuring the distance of the transducer to a reference, such as a taut metal tape.
Finally, Dahlquist U.S. Pat. No. 4,678,915 utilizes an electromagnetic signal transmitted through the sheet material for sensing the distance between traversing transducers. X and Y displacement in the plane of the moving sheet is apparently compensated for but not measured directly.